The objective of the proposed research is to study the mechanism of interference by defective influenza virus (DI virus) particles. Specifically, we proposed to determine: 1) which of the following subviral components - DI-RNP, DI-viral RNA or DI-CRNA - is the interfering molecule 2) how much information is lost in DI viral RNA compared to infectious viral RNA? 3) what is the mechanism of interference? Is it a replicative, transcriptional or translational block?